The generalized illness following, and presumably as a result of, having attended a gathering of individuals in a convention or conference setting commonly referred to as a "con", particularly when these individuals have a reputation for showering infrequently.

I woke up the day after the con with a runny nose and cough, I probably got concrud.

Illnesses which are spread around at science fiction and furry conventions due to the large numbers of people who gather and interact in one confined place. Lack of sleep, drama, stress, travel, and post con depression may also depress the immune system, contributing to con crud.

That was an awesome con I attended, pitty I caught a nasty case of con crud.